A. Field of the Invention
The present invention relates to a lock-up clutch used in a torque converter, and more particularly, a lock-up clutch having a torsion spring which enables a power input portion of the torque converter and a power output portion of the torque convertor to undergo limited relative rotary displacement with the lock-up clutch engaged therebetween.
B. Description of the Related Art
A lock-up clutch is known for use in a torque converter. Such a lock-up clutch includes a piston, a driven plate, and a torsion spring connecting the piston and the driven plate elastically in the circumferential direction. Typically, the driven plate is connected to the turbine or a hub which support the turbine. The piston can be engaged with a front cover of the torque converter through a friction element formed on the piston.
The lock-up clutch may also include a damper mechanism which is composed of a plate element and a plurality of torsion springs between the piston and the driven plate. An input element in such a damper mechanism is engaged with the piston in a manner so as to rotate with the piston by may move in the axial direction relative to the lock-up clutch.
Among damper mechanisms with such a basic structure, an intermediate member is placed between the input element and the output element, and wherein the input element and the intermediate member are connected by the first elastic member, and wherein the intermediate member and the output element is connected by the second elastic member. Since two kinds of elastic members act in series in such a mechanism, the performance of both low rigidity and wide angle of twist is obtained. In the above-mentioned structure, a relatively thick annular drive plate is used as an input element to engage the piston, a driven plate is an output element radially inside of the drive plate in the radial direction, and a pair of annular side plates as an intermediate member are placed at each axial side of the drive and driven plates.
The above-mentioned drive plate radially outward projections which engage grooves in a tube portion of the piston in a manner so as to rotate therewith but to undergo movement in the axial direction with respect thereto. Radially inward projection on the drive plate engage the first elastic member.
However, the drive plate is relatively large in size because of the configuration of the piston engagement part and the first elastic engagement part. The drive plate is also heavy because of its size and thickness. As the result, strong centrifugal forces is applied to the plate at a high speed of rotation, and the drive plate undergoes a significant level of stress and can fail or deform.
According to the structure mentioned above, the first and second coil elements are disposed in two kinds of window holes of the pair of the side plates, respectively.
There is the possibility that the first elastic elements, which is outside in the radial direction, deforms both side plates. In another words, since stronger centrifugal force is applied to the first elastic members which are located at longer distance from the center of the disk, the first elastic member presses the outer circumferential edges of the receptacles of the pair of side plates diagonally outward and in both the axial direction so as to separate both side plates. Subsequently, both side plates may be deformed, a damper mechanism may go out of order, and the side plate is in danger of being broken.